What is the best choice for mortar below a shower stall? I would think a mortar that does not crack or shrink.

Thanks,
Bill

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Bill I'll assume you mean under the shower pan (fiberglass type).....

When installing mortar under tubs and showers pans we just use Mason Mix (type used for setting bricks).

I do not want the tile to bond to the underside so we use poly as a bond breaker.

Each install depends on what the bottom looks like so we tackle each on a case by case basis.

Each manufacture will have a preferred system in place. If they have none I would look for a supplier who allows this step.

Some of the pans have large segmented sections that are hard for the mortar and air to escape when setting them in place. The key area is around the drain. Often the subfloor is not supported from below.

Sand mix is what you make a shower pan out of. If it is rich enough, it will smush, but out of the bag, it may be so coarse, it won't smush easily...think wet beach sand verses something that will flow a bit. The more water, the more likely it will shrink and crack. More cement in the mix will make it move a little easier. Not sure what their sand:cement ratio is in the prepackaged stuff...if it's in the 3:1 range, it will still pack, but have enough cement to flow a bit. A bunch of piles rather than trying to make it flat first works better.

It was a 5,000 psi mix that Sakrete recommended. It was quite thick when I put it down, but I wouldn't have wanted it so thin that it didn't stay in place.
I put down an inch layer and it was difficult to get the fiberglass base all the way down to the floor. In fact I ended up leveling the base and allowing the base to sit in the mortar about a 1/2 inch off of the subfloor. It is fully supported by all of the mortar that was put down.